A Magnetic Signature at Io: Initial Report from the Galileo Magnetometer
M. G. Kivelson, K. K. Khurana, R. J. Walker, C. T. Russell,
J. A. Linker, D. J. Southwood, C. Polanskey
During the inbound pass of the Galileo spacecraft, the magnetometer acquired 1 minute averaged measurements of the magnetic field along the trajectory as the spacecraft flew by Io. A field decrease, of nearly 40 percent of the background jovian field at closest approach to Io, was recorded. Plasma sources alone appear incapable of generating perturbations as large as those observed and an induced source for the observed moment implies an amount of free iron in the mantle much greater than expected. On the other hand, an intrinsic magnetic field of amplitude consistent with dynamo action at Io would explain the observations. It seems plausible that Io, like Earth and Mercury, is a magnetized solid planet.
Jupiter's moon Io has repeatedly surprised planetary scientists. First, Io's orbital position was unexpectedly found to control decametric radio emission from Jupiter's ionosphere (1). Early explanations suggested that the emissions were generated by magnetic field-aligned currents linking Io and Jupiter (2). These ideas were refined and linked to Alfvénic disturbances generated by the interaction of the flowing plasma of Jupiter's magnetosphere with an electrically conducting Io (3,4). After the discoveries of a large cloud of neutral sodium surrounding Io (5) and of a torus of ionized sulfur encircling Jupiter at the distance of Io's orbit (6), Voyager 1 found volcanic plumes distributed on the surface of the moon (7). The Voyager 1 magnetometer detected magnetic perturbations of ~5% of the ambient jovian magnetic field (~1900 nT) as it crossed Io's magnetic flux tube about 11 RIo (radius of Io 1821 km) below Io (8), thereby confirming the presence of a field-aligned current flowing several thousand kilometers away from the spacecraft and carrying more than 106 A into the jovian ionosphere.
The Galileo spacecraft flew by Io on 7 December 1995; its closest approach was at 17 45:58 UT (universal time) at an altitude of 898 km (9). Particles and fields data from the pass recorded on the spacecraft tape recorder will be analyzed in the early summer of 1996. However, survey data (10) read out directly from the magnetometer's (11) internal memory were returned in late December 1995. All three components of the background jovian field measured on Galileo's trajectory through the plasma torus followed predictions based on a recent extension (12) of Voyager-epoch magnetic field models (13) but in the wake of Io (that is downstream in the flow of torus plasma corotating with Jupiter), the field magnitude decreased by 695 nT in a background of 1835 nT (Fig. 1). Perturbations of the field along the spacecraft's trajectory were principally antiparallel to the model jovian field (Fig. 2). The field rotated slightly, but the bending was not what would be produced if the field had been pushed outward around Io but rather that caused by a field pulled inward toward Io. Indeed, the perturbations along the spacecraft trajectory are quite well represented by a model in which Jupiter's field is merely added to the field of an Io-centered dipole with moment aligned with the local field of Jupiter (hence antiparallel to Jupiter's dipole moment) (Fig. 3).
As the local corotation speed is greater than the Keplerian speed, the jovian plasma flows by Io at 57 km s-1. If Io is sufficiently conducting, the interaction drives currents through the plasma. Following an earlier suggestion (14), Goldreich and Lynden-Bell (2) put forward a model of the interaction of Io with the jovian magnetosphere that formed the basis for subsequent ideas. They assumed that Io's conductivity was so large that the material on the entire magnetic flux tube that threaded Io was frozen in and was dragged not only through the magnetosphere but also through the conducting ionosphere of Jupiter at the northern and southern feet of the tube. They also noted that the currents connecting to the ionosphere would be carried by Alfvén waves and that any internal Io resistivity would modify the picture. Later work by others (3, 4, 15) showed that a finite Io resistance meant that the field lines threading Io (and its immediate neighborhood) do not move precisely with Io. How effectively the field lines threading Io move with it depends on the degree of matching between the Io resistance and the net impedance of the current-carrying Alfvén waves. Normally impedances will not match and there will be slippage.
Neubauer suggested that Io could be magnetized by an internal dynamo (16). Kivelson et al. (17) added the suggestion that there might be an Io magnetosphere. There are two important consequences of Io magnetization for the form of the Io plasma interaction. Firstly, when the dipole is antialigned with the dipole moment of Jupiter reconnection will link Io's internal field to the external jovian magnetic field. Slippage between Io and the flux tubes threading it is still possible. Secondly, the effective size of the Io flux tube is increased as flux is drawn into Io from the surrounding medium (Fig. 3).
The distinction between the "slipping Io flux tube" interaction and the frozen-in tube need not be great. In the slipping case, the Io-associated perturbations are guided by a pair of opposed currents tilted at angle a = tan-t (MA)' where MA is the local Alfvén Mach number of the corotation flow (18). The frozen-in tube case corresponds to the limiting case where not only is the ambient flow stopped by the Alfvén wave perturbation but also the field attached to Io is tilted by precisely a. The attached tube is bent by the force associated with deflection of the corotating plasma around it. It is important to note that the angle between the current (which flows in the Alfvén wing) and the upstream field depends only on plasma properties and is independent of the strength of the interaction. The field perturbation and the current density can vary with the strength of the interaction, but the field cannot be tilted beyond alignment with the currents. Writing DB as the maximum transverse field perturbation in the Alfvén wing one can estimate (16, 19) I/d, the current per unit length flowing into the conductor from above or below Io, as
I/d = DB/m0 = MA/m0B = 0.4 A m-1
As double this current flows radially outward, across an object of Io's diameter, one finds the maximum current is ~3 x 106 A (20). A current of the magnitude inferred from the Voyager observations would give only ~30% of the 695 nT perturbation observed by Galileo (Fig. 1). We find (21) that a current of ~12 x 106 A is needed to produce a perturbation at Galileo's orbit of the order detected.
A magnetohydrodynamic (MHD) simulation (22, 23) of the current-carrying region as a conducting, spherically symmetric body of Io's dimensions produced a field depression similar to the actual data, but its magnitude was too small (Fig. 4). Larger perturbations are obtained if currents flow not through Io or a near-surface ionosphere but through an extended, gravitationally bound Io ionosphere that closely approaches the Galileo orbit. We examined a range of MHD simulations (22) of an unmagnetized, conducting Io and can reproduce the observed signature only if Io has a radius of 1.4 RIo, (24). There is, however, reason to doubt that a conducting ionosphere, gravitationally or collisionally bound to Io, exists at high altitude. Currents flow where ion-neutral collisions satisfy Wi = vc = nnsn with Wi, the ion gyrofrequency and vc, the ion-neutral collision frequency expressed in terms of nn, the neutral density, s, the collision cross section and n, the relative ion-neutral speed. Near Io, Wi ~ 2p s-1. With s = 4 x 10-20 m2 and n < 100 km s-1, nn> 2 x 1015 m-3, which is improbably large at 900 km altitude. Such densities are present only below ~100 km in atmospheric models (25, 26). Although Io's ionosphere does not seem capable of producing the entire perturbation, it may contribute to the asymmetry in the geometric wake.
Many theories (2, 3, 15, 27) of the Io interaction have focused on an ionospheric closure path for currents generated in the magnetospheric plasma, but newly ionized ions can also produce currents called pickup currents (28, 29). Our MHD simulations with an ionization rate of 1027 ions s-1 falling with distance as r -3.5 (and no charge exchange) show little change in the signature from the case with a conductor alone because the pickup currents are dwarfed by the ionospheric currents. Estimates suggest that charge exchange currents are not negligible but that they will not dominate unless the process occurs in localized regions. These charge exchange currents augment the Alfvén wing currents flowing towards the jovian ionosphere. Thus, the signature recorded by Voyager 1 as it crossed the Io flux tube would have included their contribution. As argued above, the inferred total current was insufficient to produce the perturbation detected by Galileo (30).
The effects of larger ionization and charge-exchange rates on the perturbation remain to be evaluated fully. The process is self-limiting as pickup and charge exchange extract energy from the flow. In the unperturbed torus plasma, the thermal energy is smaller than the flow kinetic energy, but when the flow slows, new ions may acquire thermal energy below the ambient temperature and cool the plasma. Thus pickup current density and the plasma pressure can either increase or decrease as the response is nonlinear and, therefore, hard to predict.
Finally, we consider an internally-generated magnetic field as the source of the perturbation. An internally generated field is expected to align closely with the local field of Jupiter whether the source is induced magnetization or an intrinsic field due to remanence or a self-sustained dynamo field (31). Correspondingly, Io's magnetic moment should be anti-aligned with Jupiter's (32). The fact that we do not see a change in the sign of the dominant component of the field places an upper limit of 4 x 1020 A m2 on the magnitude of the dipole moment and this is an unambiguous overestimate. Our MHD simulation of the interaction of a flowing plasma with a magnetized Io (33) gives a perturbation of the required direction, size, and spatial scale near the wake center, along the Galileo orbit (Fig. 4). At 1.5 RIo in the near-equatorial region, the field is significantly depressed. The perturhation is roughly double the drop that would be found in the absence of plasma effects, as the Alfvén wing current system and the internal magnetic field contribute in the same direction. The total current in the Alfvén wing is enhanced relative to the conducting case because the effective obstacle to the flow is larger than Io. The data depart from the model near closest approach; we suggest that local plasma structure introduces small-scale perturbations which account for the day-night asymmetry of the observations and that at less than 0.5 RIo above the surface, conductivity irregularities and higher order multipoles may also be important.
The direction of the perturbation is also that expected from an induced magnetic field in a paramagnetic Io. Near the pole, the strength of Io's field is ~1.4 times the background jovian field. To achieve this enhancement of the field strength from a paramagnetic response would not be possible with the magnetic susceptibilities (<l0-3) of typical minerals (34). However, free iron, especially iron just below its Curie point (~1000 K), exhibits a strong paramagnetic response. Tidal forcing (35) is known to heat Io's interior. Assuming that the Curie isotherm is at 100 km depth a magnetic moment per unit volume ,m ~40 A m-1 (36) is required to account for the observed perturbation. If iron, with a m = 4000 A m-1 were the source, the crust would require a volume fraction of 10-2 of free iron, which is not consistent with the expected composition of Io (37).
The possibility of an internally generated field at Io has been considered by a number of authors. Levy (31) points out that the requirements on a self-sustained dynamo are not stringent when a constant seed field acts on the body. The seed field is in the sense required to account for the observations. More recent work (38) proposes that a dynamo can develop if the system is not in thermal balance or if heat transfer in Io's mantle is chaotic with large fluctuations of heat flow in time. Nonlinear coupling between the orbital eccentricity and tidal heating can lead to an oscillatory solution with a dynamo switching on and off every few hundred million years and present roughly half the time. The magnetic moment inferred in our analysis is of the order expected from dimensional arguments and fits into a Blackett's law (39) scaling of planetary magnetic moments (Fig. 5).
Thus, unless the free iron in the mantle of Io is much greater than we expect or the plasma conditions have changed since the Voyager epoch (40), an intrinsic dynamo field of Io with a magnetic moment antialigned with Jupiter's moment and ~1020 A m2 in magnitude seems called for. This corresponds to a field strength of ~1300 nT at Io's equatorial surface in the absence of a background field. Reconnection links Io's field to Jupiter's and the foot of the Io flux tube is expected to be a distinct region in which much of the power generated by the Io interaction is dissipated in Jupiter's ionosphere, consistent with the evidence of isolated signatures at the foot of Io's flux tube (41). The interaction with a magnetized Io also perturbs the torus plasma, which means that models in which plasma currents generated near Io create the decametric arcs observed by the Voyager spacecraft (42) need not be significantly modified. The observed day-night asymmetry and the small-scale irregularities in the Io-related signature suggest that conductivity inhomogeneity, higher order magnetic multipoles, and pickup ions contribute to, without dominating, the observed magnetic perturbations. The recent evidence that Io has a large, molten iron-iron sulfide core (43) and that adequate heating to drive a dynamo field is present (31, 38) suggests that the inference of intrinsic dynamo action is physically reasonable. An intrinsic magnetic field would add Io to Earth and Mercury as the only solid planets with currently active internal dynamo fields.
REFERENCES AND NOTES
16 April 1996; accepted 2 July 1996
M. G. Kivelson, K K Khurana, R J Walker, C. T Russell, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, CA 90095-1567, USA.
J. A. Linker, science Applications international corporation, San Diego, CA 92171, USA.
D. J. Southwood, Department of Physics, Imperial College of science, Technology, and Medicine, London SW7 2BZ, UK.
C. Polanskey, Jet Propulsion Laboratory, Pasadena, CA 91109, USA.
Fig. 1. Magnetic field components and total field (nT) measured by the Calileo Magnetometer on 7 December 1995 plotted versus spacecraft event time in UT. The data are shown in right-handed System III (epoch 1965) coordinates. The dashed curves are from the model of Khurana (12). Closest approach to Io is indicated by a vertical line.
Fig. 2. Spacecraft trajectory inbound towards Jupiter in the region near Io (shaded region) The plots use a coordinate system referenced to the direction of corotation (along ). The unperturbed background field at the center of the wake lies in the x-z plane close to is positive inward towards Jupiter. (A) shows the x-y projection and indicates the flow direction The lines rooted along the trajectory are proportional to the projection of B - Bmodel of Fig. 1 and the scale for the field perturbations is indicated. Key times are given and the field data are separated by about one minute. The terminator is crossed close to the center of the wake, with the sunlit side corresponding to negative values of y. (B) shows the y-z projection of the trajectory and the perturbation field vectors. Note that the trajectory passes principally below lo's equator in this coordinate system.
Fig. 3. Field lines for vacuum superposition of a uniform field (B = 1835 nT) and a dipole field centered at Io. Here Bo is taken as 1835 nT and R is in RIo Note the bend in the field lines in the sense opposite draping around Io. Most of the flux originating within Io is directly linked to ambient flux as in a reconnection geometry.
Fig. 4. The data in the coordinate system described in Figure 2 plotted versus UT (solid curves). Also shown are the results of two MHD models (44) along the spacecraft trajectory. The short dashed line shows the model with flow past a magnetized Io (magnetic moment corresponding to a surface equatorial field of 1300 nT). The dashed curves show the model with flow past a conducting Io.
Fig. 5. The scaling relation, appropriately referred to as Blackett's law (39), between the magnetic dipole moment and the rotational angular momentum of the planets. The proposed magnetic moment of Io and the angular momentum (43) are plotted.